What's unclear is if both Zack Snyder and Joss Whedon are co-directors, but in my opinion, as long as a good movie is put out, I could care less who gets credit. Obviously they both deserve credit as Zack Snyder basically finished filming, with Joss Whedon doing script rewrites and then reshoots. The Director's Guild could also have something to do with it as contracts between the DB and studios state who gets credit, etc.

Joss Whedon is also writing, producing and directing the Batgirl movie for the DCEU.

Fueled by his restored faith in humanity and inspired by Superman’s selfless act, Bruce Wayne enlists the help of his newfound ally, Diana Prince, to face an even greater enemy. Together, Batman and Wonder Woman work quickly to find and recruit a team of metahumans to stand against this newly awakened threat. But despite the formation of this unprecedented league of heroes—Batman, Wonder Woman, Aquaman, Cyborg and The Flash—it may already be too late to save the planet from an assault of catastrophic proportions.